The present invention relates to a transistor circuit and, more particularly, to a level detecting circuit.
A level detecting circuit is generally designed to indicate the electric field strength of a signal (radio wave) in an FM/AM broadcast receiver, a satellite broadcast receiver, or the like. In general, an output from the level detecting circuit is input to a driving circuit of a signal level meter. As a means for obtaining a level detection output, a means for amplifying an RFAGC detection output from the front end portion of a tuner is available. FIG. 2 shows this means. A conventional level detecting circuit is denoted by reference numeral 15 in FIG. 2, which corresponds to an operational amplifier portion for amplifying a gain control voltage .DELTA.V of a gain control amplifier 11 of an AGCAMP.
An operation of the circuit in FIG. 2 will be described below. A second AGCAMP is constituted by the gain control amplifier 11, a post-amplifier 12, an AGC detecting circuit 13, and a DC amplifier 14. An input signal to the AGCAMP is input to the base of a transistor Q5 and is amplified by the gain control amplifier 11. The input signal is amplified by the post-amplifier 12 and is subsequently input to the AGC detecting circuit 13 to be amplitude-detected. The AGC detection output is amplified by the DC amplifier 14 and is input to a differential electronic volume control constituted by transistors Q1 to Q4 to be subjected to AGC processing. A level at the start of AGC processing is set by the AGC detecting circuit 13. Assume that a gain of the gain control amplifier 11 is to be obtained. In this case, a mutual conductance g.sub.m of a differential circuit constituted by transistors Q5 and Q6 is represented by the following equation (v.sub.T is the thermal voltage of the transistors): ##EQU1## If a current I from a constant current source for the differential circuit is temperature-corrected by using a band gap regulator to establish EQU I=v.sub.T /a (2)
then, equation (1) is rewritten as follows: ##EQU2## Therefore, a collector current I.sub.C5 of the transistor Q5 is given by the following equation, provided that an input signal to the AGCAMP is represented by v.sub.i : ##EQU3## In a differential circuit constituted by the transistors Q1 and Q2, if the base voltages of the transistors Q1 and Q2 are represented by V.sub.B1 and V.sub.B2, a voltage .DELTA.V between the bases of the transistors Q1 and Q2 is given by EQU .DELTA.V=V.sub.B2 -V.sub.B1 ( 5)
According to equations (5) and (4), a collector current I.sub.C1 of the transistor Q1 is given by ##EQU4## A substitution of equation (5) into equation (6) yields ##EQU5## Therefore, a collector voltage V.sub.C1 of the transistor Q1 is represented by equation (8): ##EQU6## An AC signal component v.sub.C1 of the collector voltage V.sub.C1 is represented by ##EQU7## That is, the input signal to the post-amplifier 12 becomes the AC signal component v.sub.C1, and an output v.sub.O from the AGCAMP is given by the following equation, provided that the gain of the post-amplifier 12 is represented by A.sub.PO : ##EQU8## If the AGCAMP is now performing an AGC operation, it can be assumed that an output from the AGCAMP is constant, and v.sub.O =V.sub.OC (11). At this time, the relationship between the input signal and the value .DELTA.V given by ##EQU9## Equation (11) is solved with respect to .DELTA.V as follows: ##EQU10## Equation (12) indicates the relationship between the input signal level v.sub.i and the difference in voltage between the transistors Q1 and Q2 in an AGCAMP operation. Referring to FIG. 2, the value .DELTA.V corresponds to an AGC detection output obtained through the DC amplifier 14. In generally, the value .DELTA.V is further amplified by an operational amplifier to obtain a level detection output. If the value .DELTA.V is amplified by the operational amplifier 15 in FIG. 2, a level detection output L.sub.O is given by ##EQU11## If equation (3) is substituted into equation (13), the level detection output L.sub.O is represented by ##EQU12##
In this conventional level detecting circuit, as indicated by equation (14): ##EQU13## since the level detection output L.sub.O includes the term of a thermal voltage, the value L.sub.O varies in relation to temperatures. For example, when a temperature t=-25.degree. C., a level detection output L.sub.1 is given by ##EQU14## When the temperature t=75.degree. C., a level detection output L.sub.2 is given by ##EQU15## (where A.sub.PO, V.sub.O, v.sub.i, and V.sub.B are constant with respect to temperatures.)
A ratio of L.sub.2 to L.sub.1 is given by ##EQU16##
That is, the level detection output varies at 40% in the temperature range of -25.degree. C. to 75.degree. C. The level detection output similarly varies with changes in the input signal v.sub.i. Since the level detection output always varies with signal changes from a low-level input to a high-level input, some kind of temperature correction is required in a level meter driving circuit to which the level detection output is input. Otherwise, the level meter varies with temperature changes.